The present invention relates to a copier, printer, facsimile transceiver or similar electrophotographic apparatus and, more particularly, to an electrophotographic process control device for allowing a stable latent image to be formed on a photoconductive drum included in the apparatus in matching relation to changes in the characteristics of the drum.
A photoconductor for implementing an electrophotographic process has the characteristics thereof relating to charging and exposure changed in a complicated way by various kinds of factors, as well known in the art. Such factors may generally be classified into three kinds, i.e., long-term deterioration factors including the wear and electrostatic fatigue of a photoconductive drum, short-term deterioration factors including fatigue due to exposure and the duration of suspension, and real-time deterioration factors including temperature. The optimal control methods for the charge potential and the amount of exposure depend on the kind of deterioration factors. It has been customary with an electrophotographic process to prepare a look-up table listing adequate control values associated with the outputs of a surface electrometer and a photosensor and determined by experiments. During actual control, the look-up table is referenced to control each parameter. A conventional PID (Proportional, Integral and Differential) system or similar control system is available for various purposes since it has a simple construction and allows control parameters thereof to be adjusted with ease. Regarding the control over a latent image, it is a common practice to measure the surface potential of the drum by a surface electrometer and set up a target potential by controlling the amount of charge and the amount of exposure.
However, the problem with the conventional control system is that the look-up table has to be prepared by a prohibitive number of experiments and, in addition, a considerable amount of data for control are necessary. Moreover, since the control is effected without specifying any of the deterioration classes, the control system has only a limited range of applicability and often causes the apparatus to run out of control. A grid, photoconductive drum and other parts included in an electrophotographic copier have their characteristics changed or deteriorated in association with the number of copies produced, temperature and humidity, toner concentration, fatigue of a developer, etc. This is apt to smear the background of a reproduction or to render the image density irregular, preventing stable image quality from being achieved. Further, in a conventional copier susceptible to the internal and external conditions as stated above, when a charge deposited on the drum by, for example, a charger using a grid voltage is partly dissipated by a laser beam issuing from a laser diode (LD) to form a latent image, it is likely that the surface potential of the photoconductor changes due to the deterioration of the drum and charger to prevent the resulting image from having a sufficient density, or that the residual potential is increased due to, for example, continuous copying to smear the background of a reproduced image. On the other hand, with the PID or similar control system, it is difficult to control a complicated object or a system whose dynamic characteristic frequently changes. The drawback with a system of the type measuring the surface potential of the photoconductive drum by an electrometer, controlling the amount of charge and the amount of exposure to produce a target image potential, and then controlling the potential of a latent image to be formed on the drum is that an exclusive latent image pattern for measurement has to be formed on the drum, slowing down, for example, the copying operation of a copier.